Polyhalogenated hydrocarbons are employed very frequently in industry and research. Thus, fluorocarbons serve as propellant gases and refrigerants and are the starting materials for the manufacture of plastics which are very chemically resistant. Chlorinated hydrocarbons are employed in great quantities as degreasing agents in metal-working plants. Other areas of application include a wide variety of chemical purifications. In addition, chlorinated hydrocarbons are the starting materials for the manufacture of polymers, pesticides, and herbicides. The polychlorinated hydrocarbons are particularly useful as heat-transfer oils or hydraulic fluids because of their high chemical and thermal resistance. The polychlorinated biphenyls (PCB) are typical representatives of this class of substances.
Recycling of spent hydrogenated hydrocarbons is exploited as far as technically possible and economically feasible. Nevertheless, about 30,000 to 40,000 tons of chlorinated hydrocarbons, which have a chlorine content &gt;20% accumulate annually in West Germany alone, even though these materials ought to be disposed of.
In addition to residues from recycling plants and production residues, there are so-called special wastes which include substances the use of which is increasingly more restricted because of safety and environmental regulations. These also must finally be disposed of. The best known examples of these special wastes are the polychlorinated biphenyls, which, in the past, have primarily been used as transformer oils and dielectrics in capacitors. In West Germany, every year for the next 10 years, it will be necessary to deal with about 6,000 tons of polychlorinated biphenyls which must be disposed of solely due to the replacement of these fluids by substitutes.
Incineration at sea is currently regarded as a major possibility for the disposal of halogenated hydrocarbons. However, international agreements (the Oslo and London Conventions), have as their object the total elimination of incineration at sea by the end of this decade. Therefore, incineration on land remains the sole alternative.
Problems are encountered when halogenated hydrocarbons, especially fluorinated and polychlorinated hydrocarbons, are incinerated in existing special waste incineration facilities. The major causes of the problems are the risk of corrosion of brick linings and waste gas lines by a high hydrogen halide (HF and HC1) concentration in the crude gas, the need to avoid emissions, especially in the incineration fluorinated hydrocarbons, and the high energy expenditure.
This disposal method is being increasingly criticized, particularly because highly toxic polychlorinated dibenzodioxins and dibenzofurans can be produced during incineration of chlorinated hydrocarbons under inadequate incineration conditions.
In Unexamined West German Patent Application No. 30 28 193 a process is disclosed for the pyrolytic degradation of halogen- and/or phosphorus-containing organic substances, in which said substances are converted after being combined with calcium oxide/calcium hydroxide at a greater-than-stoichiometric ratio and at temperatures of 300.degree. to 800.degree. C. in a reactor.
A disadvantage of this process is the fact that not all halogenated hydrocarbons can be degraded without difficulty. The temperatures necessary for the quantitative degradation of chemically and thermally very stable polyhalogenated hydrocarbons, which must include the polychlorinated biphenyls in particular, are higher than 600.degree. C. Above this temperature, mixtures of CaO and Ca(OH).sup.2 form melts with the corresponding calcium chlorides. This causes serious problems, because the required continuous passage of the solid through the reactor is hindered thereby and even rendered impossible under certain conditions. In addition to the process-engineering problems, the formation of melts concurrently results in a considerable decrease in the degradation rate for the halogenated hydrocarbons. This can be attributed to the extensive reduction in the surface of the solid reactants, which has a major effect on the reaction gas-solid reactions. Even a substantial excess of the indicated basic compounds cannot prevent melt formation with subsequent encrustation in the cool-down phase at temperatures above 600.degree. C.
Unexamined West German Patent Application No. 34 47 337 discloses a process that prevents the formation of melts in the 600-800.degree. C. temperature range by providing calcium oxide and/or calcium hydroxide in at least two fold stoichiometric excess relative to the halogen to be removed, and which the reaction mixture also contains 2 to 30% by weight of iron oxide.
A disadvantage of this process is the fact that the temperature of 800.degree. C. must not be exceeded, if incrustation is to be prevented with a high degree of certainty. However, avoidance of incrustation is a necessary prerequisite for the successful outcome of this degradation process. A temperature of 800.degree. C. is sufficient for the conversion of PCB's which are chemically and thermally very stable, but the reaction of the perhalogenated hydrocarbons with CaO is very exothermic. Thus, a considerable increase in temperature, which then must be limited to 800.degree. C. by appropriate steps, occurs in the reactor at a correspondingly higher feed rate. This increase in temperature can be reduced by partial replacement of CaO by Ca(OH).sub.2. However, water is formed from the Ca(OH).sub.2. The water in turn reacts at 800.degree. C. with the calcium chloride formed from the chlorinated hydrocarbons at 800.degree. C., converting it to hydrogen chloride.
The hydrogen chloride is thus formed is an unwanted component of the waste gas. Therefore, efforts must be made during this process to limit the reaction temperature to 800.degree. C., which in practice amounts to limiting the feed rate of the halogenated hydrocarbons.